Collation is the method by which cards are inserted into booster packs (and other products) at the factory. Multiple cards are printed together on large sheets of card stock. Common sizes for these print sheets are 11 × 11 cards and 10 × 11 cards. Sheets are then cut into individual cards.
The arrangement of cards on the print sheet is directly responsible for the order that cards appear in packs. There are different ways of doing this that have been used at different times and in different places.
Most products use multiple different print sheets. Sometimes there are practical reasons to print cards on different sheets. For example, foil cards are printed on different sheets than normal cards because they need to receive the foil treatment, and marketing (token) cards are printed on different sheets than normal cards because they use lower quality card stock. Other times, there are organizational reasons to use multiple sheets. Printing rare cards on a separate sheet makes it easy to ensure that each pack gets one rare. Sometimes additional sheets are used to add variety to the collation so that packs (or boxes) appear more random. And, finally, there is sheet math, the mathematical side of trying to fit the desired cards onto one or more sheets so that most or all of the space on the sheet is used.
Sometimes, there are extra spaces on a sheet that aren't used. These spaces might be used to print a one-off card for another purpose (like a promotional card), or they might just be discarded in the factory. Cards that are meant to be discarded in the factory are called fillers. Sometimes fillers are inserted into packs accidentally, so there is some information available about what kind of fillers have been used in real products. However, it is also possible that instead of a filler, a duplicate of another card (that is still meant to be discarded) appears on a sheet. This method prevents customers from realizing that anything has gone wrong when the filler is accidentally inserted into the pack.
For example, many sets contain 80 uncommon cards that need to be printed on 11 × 11 sheets with room for 121 cards. It would be possible just to print the cards on one sheet, but that would be wasteful, as 41 cards on the sheet would have to be fillers. The normal solution to this problem is to divide the uncommons into two groups of 40 cards. Each group of 40 cards gets its own sheet. To make good use of the available space, each card is printed three times on the sheet. (This duplication also helps makes the packs appear more random.) In this example, each sheet still has one filler, but this is much better than using only one sheet.
Most of the time, common cards get their own sheets, uncommon cards get their own sheets, basic lands get a sheet, and marketing cards get a sheet. Rares and mythic rares may be printed on the same sheet, or may get separate sheets.
If rares and mythic rares are printed on the same sheet, then each rare will be printed twice as many times on the sheet as each mythic rare. This is what makes the mythic rares more rare. This method of varying the number of times a card is printed on a sheet to create different rarities has also been used in other cases, mostly in early small sets like Arabian Nights. Rarities in such cases are labled with a letter indicating which sheet the card is from and a number indicating how many times the card appears on the sheet. For example, Arabian Nights has two sheets: an uncommon sheet and a common sheet abbreviated by "U" and "C" respectively. The most rare cards in Arabian Nights are U2 cards which appear only twice on the uncommon sheet, but there are also U3 cards which appear three times on the sheet. (Many databases translate these types of rarities into the more familiar common, uncommon, and rare buckets without further distinction. Cards that appear only a few times on the common sheet could theoretically be called uncommon or rare and cards that appear many times on the uncommon sheet could theoretically be called common.)